The present invention relates to a diagnostic apparatus for determining whether a catalyst of an internal combustion engine has deteriorated.
The principal components of a system for cleaning exhaust gases of an internal combustion engine are a catalytic converter and an air-fuel ratio feedback control unit. The catalytic converter is coupled to an exhaust pipe for use in removing pollutants such as HC, NOx and CO contained in the exhaust gases. The air-fuel ratio feedback control unit includes an O.sub.2 sensor (oxygen sensor) which is arranged on the upstream side of the catalytic converter and used for detecting an air-fuel ratio. In other words, the quantity of fuel injected to such an internal combustion engine is controlled so that the air-fuel ratio may have a predetermined (stoichiometric) value, thus enabling the engine to operate with optimum efficiency and to output the minimum amount of pollutants.
The efficiency of a conventional three-dimensional catalytic system in converting noxious components decreases as the performance of the catalytic converter itself deteriorates, even though the air-fuel ratio is precisely controlled by the air-fuel ratio feedback control unit. Therefore in order to prevent a degradation of the efficiency of the air cleaner system, it is necessary to monitor the condition of the catalytic converter so as to issue a warning when it has deteriorated. Japanese Patent Laid-Open No. 30915/1990, for example, discloses an apparatus for detecting the deterioration of a catalytic converter, entitled "Catalytic Deterioration Decision Apparatus for Internal Combustion Engine." This arrangement, includes two oxygen sensors (binary sensors), one situated on the upstream side of the catalytic converter and the other on the downstream side thereof, to measure the time lag between inversion of the output value of the upstream sensor and,that of the downstream sensor. The condition of the catalyst is determined by the scale of the time difference thus measured. More specifically, the smaller the time difference, the greater the deterioration of the catalyst.
In such catalytic deterioration detection apparatus, however, the measured time difference fluctuates as the volume of exhaust gases fluctuates, without regard to deterioration of the catalytic converter itself, which reduces the accuracy of the deterioration assessment. As a result, a catalytic converter may be determined to be free from deterioration, even though it has in fact deteriorated.